3.7 entropy and feasibility of reactions

what is entropy?

a measure of how disordered particles are in a system

a system becomes energetically more stable when it becomes more disordered

always positive

measured in JK-1mol-1

what is the entropy change equation?

describe the entropy in a solid

particles locked in a regular lattice

vibrate around a fixed position

highly ordered

low entropy state

describe the entropy in a liquid

particles have limited freedom

intermediate entropies

describe the entropy in a gas

particles free to move

highly disordered

high entropy

what with △S be for vaporisation?

always positive

what happens to entropy if number of gas molecules increase in a reaction?

entropy increases

what is Gibbs free energy △G?

measure used to predict whether a reaction is feasible

△G equation

△G = △H - T△S

S given in Jmol-1K-1 but need to convert to kJ so /1000

G in kJmol-1

H in kJmol-1

T in K

what must △G be for a reaction to be feasible?

0 or negative

are exothermic reactions spontaneous?

△H negative

usually feasible

△H - T△S negative unless △S is negative and T is large

are endothermic reactions feasible?

△H positive

may be feasible

T needs to be sufficiently large

what temperature is the reaction feasible for:

△S positive

△H negative

all temperatures

what temperature is the reaction feasible for:

△S negative

△H positive

no temperatures

what temperature is the reaction feasible for:

△S positive

△H positive

high temperatures - feasible above minimum temp

what temperature is the reaction feasible for:

△S negative

△H negative

low temperatures - feasible below maximum temp

what is △G when the reaction is at equilibrium?

△G = 0